Objectify last uses of direct db access in network/floating_ips

These were previously skipped since they're not used when all services are
on master. However, they're trivial to convert and help clean up our
reports of "what's left". Since we technically need to keep them in the
tree until we bump the network RPC version, this converts them and adds
tests.

Related to blueprint compute-manager-objects-juno

Change-Id: I17296a807524a76caa10e41ea2b05c7514169fa2
Merge "Objectify last uses of direct db access in network/floating_ips"